1 Technical Field
This disclosure relates generally to an electric motor and an electric power steering device using the same.
2 Background Art
Japanese Patent First Publication No. 2009-201255 teaches an electric motor which is used as an actuator for an electric power steering device of an automotive vehicle and equipped with a biasing member which urges an output shaft of the electric motor in an axial direction thereof. Specifically, the output shaft is retained at ends thereof by a first bearing and a second bearing. The biasing member urges the first bearing toward the second bearing, thereby pressing the output shaft in the axial direction thereof to minimize noise arising from mechanical vibration or resonance of a rotor of the electric motor in an axial direction of the rotor.
In the case where the biasing member of the electric motor is made of iron, it lead to a risk of settling thereof or a loss of a spring load produced thereby when subjected to high temperatures. In the case where the electric motor has a magnet attached to an end of the output shaft to measure, using a magnetic sensor, a magnetic flux produced by the magnet as indicating an angular position of the rotor of the electric motor, there is a risk that the biasing member made of metallic material is magnetized, which results in a disturbance to the operation of the magnetic sensor, thus leading to a decreased measurement accuracy thereof. In order to alleviate such problems, the biasing member may be made of austenite stainless steel.
The use of the austenite stainless steel for making the biasing member, however, encounters drawbacks in that residual stress remaining in the biasing member after produced or stress exerted on the biasing member when compressed may accelerate cracking due to corrosion thereof (i.e., stress corrosion cracking). In order to eliminate such a problem, a frame in which the bearings and the biasing member are disposed may be made of aluminum which is easier to corrode than stainless steel to induce sacrificial corrosion of the frame, thereby decelerating the corrosion of the biasing member to eliminate the risk of the stress corrosion cracking.
However, when the electric motor is installed on a lower portion of the body of the vehicle, it will lead to a risk that the frame become eroded by splashing with water. In order to improve corrosion resistance of the frame, a coating may be formed on a surface of the frame by means of alumite treatment, but however, it will disturb the sacrificial corrosion which minimizes the risk of the stress corrosion cracking of the frame.